<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>WeakMap选课组件</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        body {
            padding: 20px;
            width: 100vw;
            display: flex;
            box-sizing: border-box;
        }

        div {
            border: solid 2px #ddd;
            padding: 10px;
            flex: 1;
        }

        div:last-of-type {
            margin-left: -2px;
        }

        ul {
            list-style: none;
            display: flex;
            width: 200px;
            flex-direction: column;
        }

        li {
            height: 30px;
            border: solid 2px #e67e22;
            margin-bottom: 10px;
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding-left: 10px;
            color: #333;
            transition: 1s;
        }

        a {
            border-radius: 3px;
            width: 20px;
            height: 20px;
            text-decoration: none;
            text-align: center;
            background: #16a085;
            color: white;
            cursor: pointer;
            display: flex;
            justify-content: center;
            align-items: center;
            margin-right: 5px;
        }

        .remove {
            border: solid 2px #eee;
            opacity: 0.8;
            color: #eee;
        }

        .remove a {
            background: #eee;
        }

        p {
            margin-top: 20px;
        }

        p span {
            display: inline-block;
            background: #16a085;
            padding: 5px;
            color: white;
            margin-right: 10px;
            border-radius: 5px;
            margin-bottom: 10px;
        }
    </style>
</head>

<body>
    <div>
        <ul>
            <li><span>php</span><a href="javascript:;">+</a></li>
            <li><span>js</span><a href="javascript:;">+</a></li>
            <li><span>java</span><a href="javascript:;">+</a></li>
        </ul>
    </div>
    <div>
        <strong id="count">共选了2门课</strong>
        <p id="lists"></p>
    </div>
</body>
<script>
    class Lesson {
        constructor() {
            this.lis = document.querySelectorAll("ul>li");
            this.countElem = document.getElementById("count");
            this.listElem = document.getElementById("lists");
            this.map = new WeakMap();
        }
        run() {
            this.lis.forEach(item => {
                item.querySelector("a").addEventListener("click", event => {
                    const elem = event.target;
                    const state = elem.getAttribute("select");
                    if (state) {
                        elem.removeAttribute("select");
                        this.map.delete(elem.parentElement);
                        elem.innerHTML = "+";
                        elem.style.backgroundColor = "green";
                    } else {
                        elem.setAttribute("select", true);
                        this.map.set(elem.parentElement, true);
                        elem.innerHTML = "-";
                        elem.style.backgroundColor = "red";
                    }
                    this.render();
                });
            });
        }
        count() {
            return [...this.lis].reduce((count, item) => {
                return (count += this.map.has(item) ? 1 : 0);
            }, 0);
        }
        lists() {
            return [...this.lis].filter(item => {
                return this.map.has(item);
            }).map(item => {
                return `<span>${item.querySelector("span").innerHTML}</span>`;
            });
        }
        render() {
            this.countElem.innerHTML = `共选了${this.count()}课`;
            this.listElem.innerHTML = this.lists().join("");
        }
    }
    new Lesson().run();
</script>

</html>